Semiconductor laser device and method for fabricating thereof

ABSTRACT

A semiconductor laser device has on a compound semiconductor substrate at least a lower cladding layer, an active layer, an upper cladding layer and a contact layer. An upper part of the upper cladding layer and the contact layer are formed as a mesa-structured portion having a ridge stripe pattern, and the both sides of the mesa structured portion are buried with a current blocking layer. The laser device includes the current blocking layer having a pit-like recess penetrating thereof and extending towards the compound semiconductor substrate, and a portion of the recess other than that penetrating the current blocking layer being covered or buried with an insulating film or a compound semiconductor layer with a high resistivity. The compound semiconductor substrate and the electrode layer thus can be kept insulated in an area other than a current injection area, thereby non-emissive failure due to short-circuit is prevented.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 09/580,961, filed onMay 30, 2000, now U.S. Pat No. 6,654,396 which claims priority toJapanese Application No. P11-148055, filed on May 27, 1999, which areincorporated herein by reference to the extent permitted by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor laser device and amethod for fabricating thereof, and in more detail a semiconductor laserdevice having a structure capable of preventing non-emissive failure dueto short circuit and a method for fabricating such device.

2. Description of the Related Art

A visible light semiconductor laser device having a stacked structure ona GaAs substrate, wherein an active layer is sandwiched by claddinglayers made of AlGaInP or GaInP, has an oscillation wavelength between630 nm and 690 nm, and attracts a good deal of attention as a lightsource for an optical pickup used in an optical disc drive.

A structure and fabrication method of a conventional AlGaInP-basevisible light semiconductor laser device will be explained hereinafterreferring to FIG. 6. FIG. 6 shows a cross-sectional view of thesubstrate showing a structure of an AlGaInP-base semiconductor laserdevice.

An AlGaInP-base semiconductor laser device 10 has on a GaAs substrate 12a stacked structure comprises a lower cladding layer 14 made ofn-AlGaInP, an active layer 16, an upper cladding layer 18 made ofp-AlGaInP, and a contact layer 20 made of p-GaAs, and all layers areepitaxially grown in this order.

An additional semiconductor layer such as light confining layer mayoptionally be provided between the upper cladding layer 18 and thecontact layer 20. Also a buffer layer made of compound semiconductor mayoptionally be provided between the GaAs substrate 12 and the lowercladding layer 14.

Of such stacked structure, the upper cladding layer 18 and the contactlayer 20 are formed as a mesa-structured portion having a ridge stripepattern.

The both sides of the upper cladding layer 18 and the contact layer 20composing the mesa-structured portion, and the upper cladding layer 18are buried with an n-GaAs layer 22 provided as a current blocking layerto ensure current constriction, thereby a central portion of the activelayer becomes an oscillation area 15 of laser light.

A metal layer made of Au, Ni and the like, or a metal stacked film isprovided as a p-side electrode 24 on the n-GaAs layer 22 and the contactlayer 20, and as an n-side electrode 26 on the rear surface of the GaAssubstrate 12, respectively.

In order to fabricate such semiconductor laser device 10, at first thelower cladding layer 14, active layer 16, upper cladding layer 18 andcontact layer 20 are epitaxially grown in this order on the GaAssubstrate 12 by the metal-organic chemical vapor deposition (MOCVD)process.

The contact layer 20 and the upper cladding layer 18 are then etched toform the mesa-structured portion, and the n-GaAs layer 22 is thenselectively grown on the both sides of the mesa-structured portion andon the upper cladding layer 18.

Next, the p-side electrode 24 and n-side electrode 26 are formed by, forexample, the sputtering process on the outermost surface and on the rearsurface of the GaAs substrate 12.

In the process of epitaxially growing the AlGaInP layer and the like toform the stack-structured portion, there has, however, been a problem ofgenerating a growth defect in the epitaxially grown layer(s) if fineparticles of GaAs or so adhere thereon, or foreign intermediate productsare formed on the substrate during the epitaxial growth.

In the process of etching the stack-structured portion to form themesa-structured portion after the epitaxial growth, etching with an acidof such epitaxially grown layer having the growth defect will result information of a pit-like shape defect portion 28 of several to tens urndiameter reaching the GaAs substrate 12 as shown in FIG. 7, since theportion of the growth defect is labile to acid and shows a high etchrateetch rate.

If the electrode layer 24 is formed in this situation, the electrodelayer 24 intruded into the shape defect portion 28 will come intocontact with the GaAs substrate 12 to cause short circuit. Such shapedefect portion 28 can be produced in the stack-structured portion madeof compound semiconductor layers not only during the wet etching butalso during acid cleaning or alkali cleaning based on the same mechanismas described above.

As a result, short circuit will occur between currents injected to theboth electrodes, thereby current which essentially has to be injected tothe oscillation area in the active layer responsible for laseroscillation is reduced, and it causes non-emissive failures such that nolaser oscillation occurs or the laser oscillation does not continue.

It is, however, quite difficult in practice in fabricating thesemiconductor laser device to epitaxially grow the compoundsemiconductor layer after thoroughly cleaning the GaAs substrate andconfirming that no particles adhering thereon. Thus so long as thesemiconductor laser device is fabricated according to the conventionalprocess, those suffering from non-emissive failures will be more or lessproduced to degrade the production yield.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide asemiconductor laser device having a structure capable of preventingnon-emissive failure and a method for fabricating such device.

To accomplish such object, a semiconductor laser device comprises: acompound semiconductor substrate; a lower cladding layer; an activelayer; an upper cladding layer and a contact layer respectively formedon the compound semiconductor substrate, wherein an upper part of theupper cladding layer and the contact layer are formed as amesa-structured portion having a ridge stripe pattern; and a currentblocking layer having a pit-like recess penetrating thereof andextending towards the compound semiconductor substrate, the both sidesof the mesa structured portion are buried with the current blockinglayer, and a portion of the recess other than that penetrating thecurrent blocking layer being covered or buried with an insulating filmor a compound semiconductor layer with a high resistivity.

In the present invention, of the pit-like recess, a portion of whichother than that penetrating the current blocking layer is covered orburied with an insulating film or a compound semiconductor layer with ahigh resistivity, so that the compound semiconductor substrate and theelectrode layers other than the a current injection area are keptinsulated, thereby the non-emissive failures as observed for theconventional semiconductor laser device is avoided.

The pit-like recess may not necessarily reach the compound semiconductorsubstrate and may be such that penetrating the current blocking layer toreach the upper cladding layer, active layer or lower cladding layer. Itis also allowable that not only a portion of the recess other than thatpenetrating the current blocking layer, but also the entire part of therecess is covered or buried with an insulating film or a compoundsemiconductor layer with a high resistivity.

The current blocking layer is made of a compound semiconductor layerwith a high resistivity, or a current blocking layer using a p-njunction isolation.

The present invention is applicable irrespective of compositions of thecompound semiconductor substrate or compound semiconductor layers and,for example, preferably applicable to a semiconductor laser device witha laser oscillating structure composed of an AlGaInP-base or GaInP-basecompound semiconductor layer formed on a GaAs substrate. The presentinvention is applicable to both semiconductor laser devices ofedge-emitting type and surface-emitting type.

A structure responsible for the laser emission is not necessarily of thestacked structure comprising the lower cladding layer, active layer,upper cladding layer and contact layer, but also may be such structurethat having a buffer layer between the substrate and the under claddinglayer, or also may be such structure that having another layer such as alight confining layer between the contact layer and upper claddinglayer.

In a preferred embodiment of the present invention, the insulating filmmay be made of at least any one of SiO₂ film, Al₂O₃ film and SiN film, athickness of which being within a range from 100 nm to 50 μm. Theinsulating film may be a stacked film thereof.

The insulating film may be made of a semi-insulating material doped orion-implanted with boron. The compound semiconductor layer with a highresistivity may be made of a GaAs layer with a low carrier density of,for example, from 1×10¹⁶/cm³ to 1×10¹⁸/cm³, both inclusive.

One method for fabricating such semiconductor laser device (referred asa first inventive method, hereinafter) relates to a method forfabricating a semiconductor laser device having on a compoundsemiconductor substrate at least a lower cladding layer, an activelayer, an upper cladding layer and a contact layer; an upper part of theupper cladding layer and the contact layer being formed as a mesastructured portion having a ridge stripe pattern, and the both side ofthe mesa structured portion being buried with a current blocking layer,the method comprises steps of:

forming a stacked structure on a compound semiconductor substrate byepitaxially growing thereon a lower cladding layer, an active layer, anupper cladding layer and a contact layer in this order,

forming an insulating film on the entire surface of the substrateincluding the wall plane of a pit-like recess penetrating the currentblocking layer and extending towards the compound semiconductorsubstrate,

forming a photoresist film on the entire surface of the substrate;

patterning the photoresist film to form a resist mask on the insulatingfilm as well as to fill the pit-like recess with the photoresist film,

etching the insulating film using the resist mask as an etching mask toform an insulating film mask, and then etching the contact layer and theupper cladding layer using the insulating film mask as an etching maskto form a mesa-structured portion having a ridge stripe pattern,

selectively growing, using the insulating film mask as a mask, a currentblocking layer thereby to bury the both sides of the mesa-structuredportion, and

removing the insulating film mask to expose the contact layer, and thenforming an electrode layer on the surface of the substrate including onthe contact layer.

Another method for fabricating such semiconductor laser device (referredas a second inventive method, hereinafter) relates to a method forfabricating a semiconductor laser device of an edge-emitting type havingon a compound semiconductor substrate a lower cladding layer, an activelayer, an upper cladding layer and a contact layer; an upper part of theupper cladding layer and the contact layer being formed as a mesastructured portion having a ridge stripe pattern, and the both side ofthe mesa structured portion being buried with a current blocking layer,the method comprises steps of:

forming a stacked structure on a compound semiconductor substrate byepitaxially growing thereon a lower cladding layer, an active layer, anupper cladding layer and a contact layer in this order,

etching the contact layer and the upper cladding layer to form amesa-structured portion having a ridge stripe pattern,

selectively growing, using an insulating film mask, a current blockinglayer thereby to bury the both sides of the mesa-structured portion,

removing the insulating film mask to expose the contact layer, and thenforming an electrode layer on the surface of the substrate,

forming an insulating film on the entire surface of the substrateincluding the wall plane of a pit-like recess penetrating the currentblocking layer and extending towards the compound semiconductorsubstrate, and then removing the insulating film from an area other thanthe wall plane of the pit-like recess, and

forming an electrode layer on the surface of the substrate including onthe contact layer.

Still another method for fabricating such semiconductor laser device(referred as a third inventive method, hereinafter) relates to a methodfor fabricating a semiconductor laser device of an edge-emitting typehaving on a compound semiconductor substrate a lower cladding layer, anactive layer, an upper cladding layer and a contact layer; an upper partof the upper cladding layer and the contact layer being formed as a mesastructured portion having a ridge stripe pattern, and the both side ofthe mesa structured portion being buried with a current blocking layer,the method comprises steps of:

forming a stacked structure on a compound semiconductor substrate byepitaxially growing thereon a lower cladding layer, an active layer, anupper cladding layer and a contact layer in this order,

etching the contact layer and the upper cladding layer to form amesa-structured portion having a ridge stripe pattern,

selectively growing, using an insulating film mask, a current blockinglayer with a low carrier density thereby to bury the both sides of themesa-structured portion and a pit-like recess extending towards thecompound semiconductor substrate, and then removing the insulating filmmask to expose the contact layer, and

forming an electrode layer on the surface of the substrate including thecontact layer.

Still further another method for fabricating such semiconductor laserdevice (referred as a fourth inventive method, hereinafter) relates to amethod for fabricating a semiconductor laser device of an edge-emittingtype having on a compound semiconductor substrate a lower claddinglayer, an active layer, an upper cladding layer and a contact layer; anupper part of the upper cladding layer and the contact layer beingformed as a mesa structured portion having a ridge stripe pattern, andthe both side of the mesa structured portion being buried with a currentblocking layer, the method comprises steps of:

forming a stacked structure on a compound semiconductor substrate byepitaxially growing thereon a lower cladding layer, an active layer, anupper cladding layer and a contact layer in this order,

etching the contact layer and the upper cladding layer to form amesa-structured portion having a ridge stripe pattern,

selectively growing, using an insulating film mask, a current blockinglayer thereby to bury the both sides of the mesa-structured portion, andthen removing the insulating film mask to expose the contact layer,

forming a resist pattern on the contact layer, and performing ionimplantation to the entire surface of the substrate thereby to convertthe outermost surface of the wall plane of a pit-like recess penetratingthe current blocking layer and extending towards the compoundsemiconductor substrate into a layer with a higher resistivity, and

removing the resist pattern thereby to form an electrode layer on thesurface of the substrate including on the contact layer withoutannealing.

While there is no specific limitation on a method for forming theinsulating film in the first to fourth inventive methods, the film ispreferably formed by the chemical vapor deposition (CVD) process. Thecurrent blocking layer is formed by the metal-organic chemical vapordeposition (MOCVD) process.

There is no specific limitation on ion species in the fourth inventivemethod, and boron can be ion-implanted for example.

In the first, second and fourth inventive methods, the wall plane of thepit-like recess conceptually include a bottom plane of the recess, aswell as a side plane thereof.

According to the present invention, in the process of fabricating thesemiconductor laser device, at least a portion excluding such thatpenetrating the current blocking layer of the pit-like recess, occurredso as to penetrate the current blocking layer and to reach the compoundsemiconductor substrate, is covered or filled with the insulating filmor the compound semiconductor layer with a higher resistivity, so thatthe compound semiconductor substrate and the electrode layer can be keptinsulated in an area other than a current injection area, therebynon-emissive failure as has been observed in the conventionalsemiconductor laser device is preventted.

The method according to the present invention embodies a preferablemethod for fabricating the semiconductor laser device of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a to 1 c are sectional views showing layer structurescorresponded to the individual process steps for fabricating asemiconductor laser device according to Example 1;

FIGS. 1 d to 1 f are sectional views showing, as continued from FIG. 1c, layer structures corresponded to the individual process steps forfabricating a semiconductor laser device according to Example 1;

FIGS. 2 a to 2 c are sectional views showing layer structurescorresponded to the individual process steps for fabricating asemiconductor laser device according to Example 2;

FIGS. 2 d and 2 e are sectional views showing, as continued from FIG. 2c, layer structures corresponded to the individual process steps forfabricating a semiconductor laser device according to Example 2;

FIGS. 3 a to 3 c are sectional views showing layer structurescorresponded to the individual process steps for fabricating asemiconductor laser device according to Example 3;

FIGS. 3 d to 3 f are sectional views showing, as continued from FIG. 3c, layer structures corresponded to the individual process steps forfabricating a semiconductor laser device according to Example 3;

FIGS. 4 a to 4 c are sectional views showing layer structurescorresponded to the individual process steps for fabricating asemiconductor laser device according to Example 4;

FIG. 5 is a sectional view showing, as continued from FIG. 4 c, layerstructures corresponded to a process step for fabricating asemiconductor layer device according to Example 4;

FIG. 6 is a sectional view of a substrate showing a conventional visiblelight semiconductor laser device; and FIG. 7 is a sectional view of asubstrate for explaining the pit-like recess.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention will be explainedspecifically and in detail referring to the attached drawingshereinafter.

EXAMPLE 1

This Example relates to one embodiment of a semiconductor laser deviceof the present invention, and such laser device that obtained by afabrication method according to the first inventive method. FIGS. 1 a to1 c, and FIGS. 1 d to 1 f are sectional views showing layer structurescorresponded to the individual process steps for fabricating asemiconductor laser device according to this Example.

In a semiconductor laser device 38 of Example 1, as shown in FIG. 1 f, apit-like recess 30 is formed penetrating an n-GaAs layer 22 (currentblocking layer) to reach a GaAs substrate 12, and a wall planecorresponded to a portion of the recess penetrating an upper claddinglayer 18, an active layer 16 and a lower layer 14 is covered with aninsulating film 32.

According to the semiconductor laser device 38 of Example 1, shortcircuit is avoided since the pit-like recess 30 is covered with theinsulating film 32, and such insulating film 32 is eventually interposedbetween an electrode layer 37 and the GaAs substrate 12. For this, theGaAs substrate 12 and the electrode layer 37 can be kept insulated in anarea other than a current injection area, thereby non-emissive failureas has been observed in the conventional semiconductor laser device isprevented.

Next, a method for fabricating the semiconductor laser device 38according to Example 1 will be explained referring to FIGS. 1 a to 1 f.

In this Example at first, similarly to the conventional process, thelower cladding layer 14 made of n-AlGaInP, the active layer 16, theupper cladding layer 18 made of p-AlGaInP and the contact layer 20 madeof p-GaAs are epitaxially grown on the GaAs substrate 12 in this orderby, for example, the metal-organic chemical vapor deposition (MOCVD)process to form the stacked structure, as shown in FIG. 1 a.

Cleaning of such stacked structure using acid or alkali may in somecases result in formation of the pit-like recess 30 of several to tensμm diameter penetrating the contact layer 20, upper cladding layer 18,active layer 16 and lower cladding layer 14 and reaching the GaAssubstrate 12, due to the above-described growth defect formed during theepitaxial growth, as shown in FIG. 1 a.

Therefore in the present Example, the insulating film 32 of 50 μm thickmade of SiO₂ film, Al₂O₃ film or SiN film for forming a mask for theselective growth is formed on the entire surface of the substrate so asto cover also inner surface of the pit-like recess 30 as shown in FIG. 1b.

Next, a photoresist film 34 is formed on the insulating film 32 and isthen processed to form a resist mask 34 for patterning the insulatingfilm 32 as shown in FIG. 1 c. At this time, also the pit-like recess 30is filled with the photoresist film 34 as shown in FIG. 1 c.

The insulating film 32 is then patterned using the resist mask 34 so asto leave the insulating film 32 in the pit-like recess 30 as well as toform an insulating film mask 36 used for the etching and selectivegrowth as shown in FIG. 1 d.

After the resist mask 34 is removed, the contact layer 20 and uppercladding layer 18 are etched by the wet etching process using a mixedsolution of sulfuric acid and hydrogen peroxide as an etchant to formthe mesa-structured portion having a ridge stripe pattern.

The n-GaAs layer 22 as a current blocking layer is then selectivelygrown by the selective growth process using the insulating film mask 36as shown in FIG. 1 e.

The insulating film mask 36 formed on the contact layer 20 and used forthe selective growth is then removed, and the electrode layer 37 servesas a p-side electrode is formed as shown in FIG. 1 f. Thus thesemiconductor laser device 38 can be fabricated according to the presentExample.

EXAMPLE 2

This Example relates to another embodiment of a semiconductor laserdevice of the present invention, and such laser device that obtained bya fabrication method according to the second inventive method. FIGS. 2 ato 2 c, and FIGS. 2 d and 2 e are sectional views showing layerstructures corresponded to the individual process steps for fabricatinga semiconductor laser device according to this Example.

In a semiconductor laser device 48 of Example 2, as shown in FIG. 2 e, awall plane of a pit-like recess 40, occurred so as to penetrate then-GaAs layer 22 (current blocking layer), upper cladding layer 18,active layer 16 and lower layer 14, and to reach the GaAs substrate 12,is covered with an insulating film 42.

According to the semiconductor laser device 48 of Example 2, shortcircuit is avoided since the pit-like recess 40 is covered with theinsulating film 42, and such insulating film 42 is eventually interposedbetween an electrode layer 46 and the GaAs substrate 12.

For this, the GaAs substrate 12 and the electrode layer 46 can be keptinsulated in an area other than a current injection area, therebynon-emissive failure as has been observed in the conventionalsemiconductor laser device is prevented.

Next, a method for fabricating the semiconductor laser device 48according to Example 2 will be explained referring to FIGS. 2 a to 2 e.

In this Example at first, similarly to the conventional process, thelower cladding layer 14 made of n-AlGaInP, the active layer 16, theupper cladding layer 18 made of p-AlGaInP and the contact layer 20 madeof p-GaAs are epitaxially grown on the GaAs substrate 12 in this orderby, for example, the metal-organic chemical vapor deposition (MOCVD)process to form the stacked structure, as shown in FIG. 2 a.

Next, the contact layer 20 and upper cladding layer 18 are etched by thewet etching process using a mixed solution of sulfuric acid and hydrogenperoxide as an etchant to form the mesa-structured portion, and then-GaAs layer 22 is then selectively grown on the upper cladding layer 18and on the both sides of the mesa-structured portion.

In the stacked structure thus processed, the pit-like recess 40 ofseveral to tens μm diameter may in some cases occur so as to penetratethe n-GaAs layer 22, upper cladding layer 18, active layer 16 and lowercladding layer 14 and reaching the GaAs substrate 12, due to theabove-described growth defect formed during the epitaxial growth, asshown in FIG. 2 a.

Therefore in the present Example 2, the insulating film 42 of 50 μmthick made of SiO₂ film or Al₂O₃ film is formed on the entire surface ofthe substrate so as to cover also inner surface of the pit-like recess40 as shown in FIG. 2 b.

Next, a photoresist film 44 is formed on the entire surface of thesubstrate so as to also fill the pit-like recess 40, and the photoresistfilm 44 is then removed in an area exclusive of the filled portion inthe pit-like recess 40.

The insulating film 42 is then removed using the photoresist mask 44 inan area exclusive of that corresponded to the pit-like recess 40 therebyto expose the contact layer 20 and n-GaAs layer 22 as shown in FIG. 2 d.

The photoresist film 44 is then removed, and an electrode layer 46serves as a p-side electrode is formed on the entire surface of thesubstrate as shown in FIG. 2 e. Thus the semiconductor laser device 48can be fabricated according to the present Example.

EXAMPLE 3

This Example relates to still another embodiment of a semiconductorlaser device of the present invention, and such laser device thatobtained by a fabrication method according to the third inventivemethod. FIGS. 3 a to 3 c, and FIGS. 3 d to 3 f are sectional viewsshowing layer structures corresponded to the individual process stepsfor fabricating a semiconductor laser device according to this Example.

In a semiconductor laser device 59 of Example 3, as shown in FIG. 3 f, awall plane of a pit-like recess 50, occurred so as to penetrate theupper cladding layer 18, active layer 16 and lower layer 14, and toreach the GaAs substrate 12, is filled with the n-GaAs layer 22 with alow carrier density of, for example, 1×10¹⁸/cm³ or less.

According to the semiconductor laser device 59 of Example 3, shortcircuit is avoided since the pit-like recess 50 is filled with then-GaAs layer 22 with a low carrier density, and such n-GaAs layer 22 iseventually interposed between an electrode layer 58 and the GaAssubstrate 12.

For this, the GaAs substrate 12 and the electrode layer 58 can be keptinsulated in an area other than a current injection area, therebynon-emissive failure as has been observed in the conventionalsemiconductor laser device is prevented.

Next, a method for fabricating the semiconductor laser device 59according to Example 3 will be explained referring to FIGS. 3 a to 3 f.

In this Example at first, similarly to the conventional process, thelower cladding layer 14 made of n-AlGaInP, the active layer 16, theupper cladding layer 18 made of p-AlGaInP and the contact layer 20 madeof p-GaAs are epitaxially grown on the GaAs substrate 12 in this orderby, for example, the metal-organic chemical vapor deposition (MOCVD)process to form the stacked structure, as shown in FIG. 3 a.

Cleaning of such stacked structure using acid or alkali may in somecases result in formation of the pit-like recess 50 of several to tensμm diameter penetrating the contact layer 20, upper cladding layer 18,active layer 16 and lower cladding layer 14 and reaching the GaAssubstrate 12, due to the above-described growth defect formed during theepitaxial growth, as shown in FIG. 3 a.

Next, the insulating film 52 of 50 μm thick made of SiO₂ film or Al₂O₃film for forming a mask for the etching and selective growth is formedon the entire surface of the substrate as shown in FIG. 3 b.

The photoresist film 54 is then formed on the insulating film 52 and isthen processed to form a resist mask 54 for patterning the insulatingfilm 52 as shown in FIG. 3 c.

At this time in Example 3, photoresist film 54 is etched based onetching conditions not allowing the photoresist film 54 remain in thepit-like recess 50, unlike Example 1.

The insulating film 52 is then etched using the photoresist mask 54 toform an insulating film mask 56 as shown in FIG. 3 d.

Next, the contact layer 20 and upper cladding layer 18 are etched usingthe insulating film mask 56 to form the mesa-structured portion, and then-GaAs layer 22 with a low carrier density of, for example, 1×10¹⁸/cm³or below is then grown by the selective growth process using the mask 56to fill the both sides of the mesa-structured portion and the pit-likerecess 50 as shown in FIG. 3 e.

The insulating film mask 56 is then removed to expose the contact layer20, and the electrode layer 58 serves as a p-side electrode is thenformed on the n-GaAs layer 22 and contact layer 20 by, for example, thesputtering process. Thus the semiconductor laser device 59 can befabricated according to the present Example.

EXAMPLE 4

This Example relates to still further another embodiment of asemiconductor laser device of the present invention, and such laserdevice that obtained by a fabrication method according to the fourthinventive method. FIGS. 4 a to 4 c are sectional views showing layerstructures corresponded to the individual process steps for fabricatinga semiconductor laser device according to this Example.

In a semiconductor laser device 66 of Example 4, as shown in FIG. 5 anoutermost surface of a wall plane of a pit-like recess 40, occurred soas to penetrate the n-GaAs layer 22 (current blocking layer), uppercladding layer 18, active layer 16 and lower layer 14, and to reach theGaAs substrate 12, is converted into a layer 62 with a higherresistivity by ion implantation of boron.

According to the semiconductor laser device 66 of Example 4, shortcircuit is avoided since the layer 62 having a higher resistivity isprovided between the an electrode layer 64 and the GaAs substrate 12.For this, the GaAs substrate 12 and the electrode layer 64 can be keptinsulated in an area other than a current injection area, therebynon-emissive failure as has been observed in the conventionalsemiconductor laser device is prevented.

Next, a method for fabricating the semiconductor laser device 66according to Example 4 will be explained referring to FIGS. 4 a to 5.

In this Example at first, similarly to Example 2, the lower claddinglayer 14 made of n-AlGaInP, the active layer 16, the upper claddinglayer 18 made of p-AlGaInP and the contact layer 20 made of p-GaAs areepitaxially grown on the GaAs substrate 12 in this order by, forexample, the metal-organic chemical vapor deposition (MOCVD) process toform the stacked structure, as shown in FIG. 4 a.

Next, the contact layer 20 and upper cladding layer 18 are etched by thewet etching process using a mixed solution of sulfuric acid and hydrogenperoxide as an etchant to form the mesa-structured portion, and then-GaAs layer 22 is then selectively grown on the upper cladding layer 18and on the both sides of the mesa-structured portion.

In this stage after such processes are completed, the pit-like recess 40of several to tens μm diameter may in some cases occur so as topenetrate the n-GaAs layer 22, upper cladding layer 18, active layer 16and lower cladding layer 14 and reaching the GaAs substrate 12, due tothe above-described growth defect formed during the epitaxial growth, asshown in FIG. 4 a.

Therefore in the Example 4, a photoresist film is formed on the entiresurface of the substrate, and the film is then patterned to form aresist mask 60 covering at least the contact layer 20 while exposing anarea including the pit-like recess 40 as shown in FIG. 4 b.

Ions, for example boron ions, are then implanted according to theconditions shown below using the resist mask 60 as a mask as shown inFIG. 4 c thereby to convert the outermost surfaces of the GaAs substrate12, lower cladding layer 14, active layer 16, upper cladding layer 18and n-GaAs layer 22 into a layer 62 with a higher resistivity:

Implantation energy: 140 keV

Dose amount: 7×10¹⁴/cm²

The resist mask 60 is then removed to expose the contact layer 20, andthe electrode layer 64 serves as a p-side electrode is then formed onthe n-GaAs layer 22 and contact layer 20 by, for example, the sputteringprocess without performing annealing for the ion implanted surfaces.Thus the semiconductor laser device 66 can be fabricated according tothe present Example.

1. In a method for fabricating a semiconductor laser device having on acompound semiconductor substrate at least a lower cladding layer, anactive layer, an upper cladding layer and a contact layer; an upper partof the upper cladding layer and the contact layer being formed as a mesastructured portion having a ridge stripe pattern, and the both side ofthe mesa structured portion being buried with a current blocking layer,the method comprising steps of: forming a stacked structure on acompound semiconductor substrate by epitaxially growing thereon a lowercladding layer, an active layer, an upper cladding layer and a contactlayer in this order, forming an insulating film on the entire surface ofthe substrate including the wall plane of a pit-like recess penetratingthe current blocking layer and extending towards the compoundsemiconductor substrate, forming a photoresist film on the entiresurface of the substrate, patterning the photoresist film to form aresist mask on the insulating film as well as to fill the pit-likerecess with the photoresist film, etching the insulating film using theresist mask as an etching mask to form an insulating film mask, and thenetching the contact layer and the upper cladding layer using theinsulating film mask as an etching mask to form a mesa-structuredportion having a ridge stripe pattern, selectively growing, using theinsulating film mask as a mask, a current blocking layer thereby to burythe both sides of the mesa-structured portion, and removing theinsulating film mask to expose the contact layer, and then forming anelectrode layer on the surface of the substrate including on the contactlayer.
 2. In a method for fabricating a semiconductor laser device of anedge-emitting type having on a compound semiconductor substrate a lowercladding layer, an active layer, an upper cladding layer and a contactlayer; an upper part of the upper cladding layer and the contact layerbeing formed as a mesa structured portion having a ridge stripe pattern,and the both side of the mesa structured portion being buried with acurrent blocking layer, the method comprising steps of: forming astacked structure on a compound semiconductor substrate by epitaxiallygrowing thereon a lower cladding layer, an active layer, an uppercladding layer and a contact layer in this order, etching the contactlayer and the upper cladding layer to form a mesa-structured portionhaving a ridge stripe pattern, selectively growing, using an insulatingfilm mask, a current blocking layer thereby to bury the both sides ofthe mesa-structured portion, removing the insulating film mask to exposethe contact layer, and then forming an electrode layer on the surface ofthe substrate, forming an insulating film on the entire surface of thesubstrate including the wall plane of a pit-like recess penetrating thecurrent blocking layer and extending towards the compound semiconductorsubstrate, and then removing the insulating film from an area other thanthe wall plane of the pit-like recess, and forming an electrode layer onthe surface of the substrate including on the contact layer.
 3. In amethod for fabricating a semiconductor laser device of an edge-emittingtype having on a compound semiconductor substrate a lower claddinglayer, an active layer, an upper cladding layer and a contact layer; anupper part of the upper cladding layer and the contact layer beingformed as a mesa structured portion having a ridge stripe pattern, andthe both side of the mesa structured portion being buried with a currentblocking layer, the method comprising steps of: forming a stackedstructure on a compound semiconductor substrate by epitaxially growingthereon a lower cladding layer, an active layer, an upper cladding layerand a contact layer in this order, etching the contact layer and theupper cladding layer to form a mesa-structured portion having a ridgestripe pattern, selectively growing, using an insulating film mask, acurrent blocking layer with a low carrier density thereby to bury theboth sides of the mesa-structured portion and a pit-like recessextending towards the compound semiconductor substrate, and thenremoving the insulating film mask to expose the contact layer, andforming an electrode layer on the surface of the substrate including thecontact layer.
 4. In a method for fabricating a semiconductor laserdevice of an edge-emitting type having on a compound semiconductorsubstrate a lower cladding layer, an active layer, an upper claddinglayer and a contact layer; an upper part of the upper cladding layer andthe contact layer being formed as a mesa structured portion having aridge stripe pattern, and the both side of the mesa structured portionbeing buried with a current blocking layer, the method comprising stepsof: forming a stacked structure on a compound semiconductor substrate byepitaxially growing thereon a lower cladding layer, an active layer, anupper cladding layer and a contact layer in this order, etching thecontact layer and the upper cladding layer to form a mesa-structuredportion having a ridge stripe pattern, selectively growing, using aninsulating film mask, a current blocking layer thereby to bury the bothsides of the mesa-structured portion, and then removing the insulatingfilm mask to expose the contact layer, forming a resist pattern on thecontact layer, and performing ion implantation to the entire surface ofthe substrate thereby to convert the outermost surface of the wall planeof a pit-like recess penetrating the current blocking layer andextending towards the compound semiconductor substrate into a layer witha higher resistivity, and removing the resist pattern thereby to form anelectrode layer on the surface of the substrate including on the contactlayer without annealing.